In this circuit, the BC547 is used. We have attached resistors of 100 Ohm at the base of transistors Q1, Q2 Q3 to protect the transistor from burning from high current across the LEDs. When there is no current on the base of transistors current does not flows from emitter to collector that is way LED will remain OFF.
When the water level reaches the point A, a very small current (conductive current) is created between the base of transistor Q1 and the positive terminal of the battery. Therefore, the Collector and Emitter of Q1 connected. As soon as the positive voltage is applied at the base of transistor Q1, the transistor Q1 is turned on and the RED LED starts glowing. Similar all transistors turned on as conductive current from the base is applied with water. All transistors turned on when the tank is full. The buzzer will be turned on as Q4 turned on which indicates that the tank is full.
Features of Water Level Indicator Circuit using transistors: